The development ofGlossiphonia complanatatakes place in about 40 days. The young are attached to the parent's ventral side by means of their posterior sucker. At the time of hatching muscle cells are not recognizable. Therefore, the various morphogenetic steps of these round circomyarian fibers can be analyzed. The contractile material starts to be formed in a continuous peripheral ring encircling a central cytoplasmic core. Thus the round circomyarian condition is achieved from the very beginning. A comparison is made with the morphogenesis of muscle fibers in oligochaetes where an initial platimyarian condition is present. The formation of the gut in glossiphonids is completed in the last phases of embryonic development. When the young individual detaches from the parent some yolk store is still present in the gut diverticula. This condition is related to the peculiar trophic dependence of the young on the parent.